The present invention relates generally to a surface treatment for stainless steel, and more particularly to a treatment for preparing a stainless steel surface for bonding to an adhesive.
Adhesively bonded stainless steel components often fail at the interface between the adhesive and the oxide layer at the surface of the stainless steel, or within the oxide layer itself. The surface of a stainless steel part that is to be bonded to an adhesive must therefore be particularly treated to impart certain characteristics to it prior to contact with the adhesive if a good, lasting bond is to be formed therewith. The stainless steel surface must be clean and smut-free, because any dirt, grease, smut, or other impurity will interfere with a proper bond being made. The surface must also be properly passivated, meaning it must be inert to the surrounding environment, have the proper thickness, be well-adhered to the base stainless steel, and provide durable adhesion sites for attachment to the adhesive. An unstable or insufficiently passivated surface is subject to moisture adsorption, particularly when the bond is subjected to cyclic stresses from thermal or mechanical loads. This leads to passivation breakdown and corrosion. A third characteristic important to good bonding is a degree of surface roughness sufficient to provide good mechanical interlocking with the adhesive.